Add yajl as a deve dependency

We use it for the benchmarks, and it won't load if we haven't told
bundler we want it.

Gemfile.lock

@@ -11,6 +11,7 @@ GEM
     rake-compiler (0.9.9)
       rake
     thoughtbot-shoulda (2.11.1)
+    yajl (0.3.4)
 
 PLATFORMS
   ruby
@@ -21,6 +22,7 @@ DEPENDENCIES
   rake
   rake-compiler (~> 0.9.0)
   thoughtbot-shoulda
+  yajl
 
 BUNDLED WITH
-   1.12.4
+   1.12.5

bert.gemspec

@@ -25,4 +25,5 @@ Gem::Specification.new do |s|
     s.add_development_dependency "git"
     s.add_development_dependency "rake"
     s.add_development_dependency "rake-compiler", "~> 0.9.0"
+    s.add_development_dependency "yajl"
 end